1. Field of the Invention
The present invention relates to a slicing ratio controlling circuit for controlling a ratio of a slice level used for binarization of a signal, more particularly relates to a slicing ratio controlling circuit for controlling a ratio of a slice level and an input signal for use for binarization of a signal in a bar code reader etc.
In general, a bar code reader illuminates a bar code, detects the reflected light, and recognizes the black bars and white bars making up the bar code in accordance with the brightness. Since the brightness changes by a large degree at the juncture of the black bars and white bars, the point of change is used to find the juncture between black bars and white bars and thereby detect them.
2. Description of the Related Art
As will be explained in detail later with reference to the drawings, the amount of the reflected light received from the portions corresponding to the white bars is large, while the amount of the reflected light received from the portions corresponding to the black bars is small. Therefore, it is possible to compare the light reflected from the bar code with the magnitude of a threshold signal (slice level) to recognize white and black bars.
The amount of light reflected from a bar code changes tremendously depending on the distance between the bar code and the bar code reader and the angle between the bar code and the illuminated light (reflected light). In some cases, the amount of reflected light from the bar code received by the bar code reader may become extremely small. Therefore, if the slice level is set to a fixed value and if the amount of light reflected from the bar code is small, there is a chance that the reflected light will always be recognized as being smaller than the slice level. Conversely, if the amount of light reflected from the bar code is extremely large, there is a chance that the reflected light will always be larger than the slice level.
To solve this problem, the previous practice has been to generate the slice level based on the amount of the reflected light received by the bar code reader (signal output by photodetector). Use of this technique enables the slice level to be changed in accordance with the amount of reflected light received by the bar code reader and therefore the above problem to be eliminated. This slice level signal is generated by inputting the signal output from the photodetector into a peak holding circuit. The magnitude of the slice level signal is set to have a specific ratio with respect to the amount of the received light.
If the amount of the light reflected from the bar code is small, however, there is a chance that noise may become noticeable in the signal from the photodetector. On the other hand, if the slice level signal is generated from the peak holding circuit, there is a chance that, depending on the configuration of the peak holding circuit, the magnitude of the slice level signal generated by it will not become the envisioned ratio with respect to the magnitude of the signal to be sliced but will become smaller than this.
If this situation arises, then when comparing the signal to be sliced and the slice level signal for binarization of the input signal and detection of the white bars and black bars, there is the problem that the noise component superposed on the signal from the photodetector will exceed the slice level signal and result in erroneous detection. This tendency becomes more marked when the amount of the light reflected from the bar code is small.